1. Field of The Invention
This invention relates to a lubricant oil composition, more particularly, the composition characterized by high transmission torque capacity and good anti-shudder property, and useful for an automatic and continuously variable transmission (e.g., belt or traction type) with a built-in torque converter equipped with a lock-up clutch, the transmission being used in an automobile.
2. Description of The Related Art
A lubricant oil for an automatic or continuously variable transmission can be used in an automobile equipped with a torque converter, gear and hydraulic mechanisms, wet type clutch, and so on. This type of lubricant is required to have a variety of properties, because it functions as the power transmission medium for the torque converter, and hydraulic and control systems; as the lubricant and temperature-controlling medium for the gears, bearings, wet type clutch, and so on; and as the lubricant medium and the medium to maintain the function-related characteristics of the friction material, in order to smoothly operate the transmission.
Recently, lock-up clutches have been built in torque converters in many automobiles to improve mileage. In this mechanism, a transmission is built in a torque converter. A lock-up clutch is a device that directly transmits the engine driving force to the transmission under varying running conditions. Torque converter efficiency can be enhanced, when switching between torque converter driving and direct driving is well timed.
However, a conventional lock-up clutch mechanism works only in a high speed range, and not in a low speed range where engine torque widely varies.
Recently, slip control is adopted to help the lock-up clutch work in the low speed range of an automatic transmission. However, abnormal vibration of the car body, known as shudder, has been frequently observed at the lock-up clutch's surface, when the lock-up mechanism is operated in a low speed range. Such a phenomenon is more pronounced, when coefficient of friction decreases as relative sliding velocity increases at the slip-controlled lock-up clutch. In order to prevent the shudder phenomenon, the lubricant is required to have good .mu.(coefficient of friction)-V (sliding velocity) characteristics. In other words, it is required to have a coefficient of friction which increases as sliding velocity increases, i.e., positive .mu.-V relationship, for an automatic and continuously variable transmission.
Esters of phosphates, aliphatic acids and fatty amides have been proposed as friction modifiers for automatic transmission lubricants, as disclosed by Japanese Laid-open Patent application No. 63-254196. However, these modifiers have disadvantages which result in a decreasing coefficient of friction at the lock-up clutch in a low sliding velocity range, and in insufficient transmission torque when the clutch is connected.
Use of metallic detergents and ashless depressants has been proposed, as disclosed by Japanese Laid-open Patent application Nos. 5-105892, 6-271883, 8-127789 and 8-319494, to increase transmission torque capacity. Nevertheless, however, few lubricants have exhibited sufficient friction-related properties, even in the presence of the above additives.
As discussed above, the .mu.-V characteristics trade off with transmission torque capacity, and it is necessary to increase transmission torque capacity while keeping a positive .mu.-V relationship. As a result, there have been demands increasingly for the technique to increase coefficient of friction in a high sliding velocity.